Polycomb group (PcG) proteins perpetuate a state of repression of genes through cell division. The polycomb responsive element (PRE) is a DNA sequence that is required for the action of PcG. The PRE provides a nucleation centre from which PcG proteins propagate an inactive structure. The trithorax group (trxG) proteins antagonize the action of PcG. The PcG and trxG can bind to the same PRE sequence with opposing effects.
The PcG proteins have some common regulatory roles. They function in large complexes; for example, the polycomb repressive complex (PRC1) contains Pc, several PcG proteins and five general transcription factors. They affect chromatin remodelling and induce repression. Once repression is established, the PcG proteins recognize it and perpetuate it through the cell division of the daughter cells.
The PRE is a complex structure of about 10 Kb. Several proteins have binding sites within the PRE sequence; for example, PcG, GAGA factor, etc. Once PcG binds the PRE, which provides a state of nucleation centre, a structural state depending on PcG proteins propagate.
The trxG proteins have the opposite effect to the PcG proteins. They maintain genes in an active state. They are quite diverse. Some comprise subunits of chromatin remodelling enzymes such as SWI/SNF, whereas others have histone-modifying activities. trxG also binds to the same PRE sequence to which PcG binds. The trxG proteins act by making chromatin continuously accessible to transcription factors. Both PcG and trxG can regulate homeotic gene promoters some distance away from PRE through DNA looping.